EX Champions' Ballad is a main quest from The Legend of Zelda: Breath of the Wild. It is unlocked upon installation of the DLC Pack "The Champions' Ballad". After freeing all the Divine Beasts, Link receives a message from Princess Zelda requesting him to return to the Shrine of Resurrection. Here he obtains the One-Hit Obliterator and is tasked with using it to defeat four groups of monsters. Defeating each group of enemies causes a Shrine to appear. Upon completing all four Shrines, the One-Hit Obliterator will burst into four different-colored orbs that soar through the sky to land relatively near the towns of each Champion.
At each of these four locations, Link meets Kass and learns more about his teacher's unfinished song about the four Champions, "The Champions' Ballad". The four main quests "EX Champion Daruk's, Mipha's, Revali's and Urbosa's Songs" are triggered at the respective locations. Each part of the song mentions three trials the respective Champion also had to overcome. Upon completing all three of a Champion's trials, Link is summoned to their respective Divine Beast. From there, he enters the Illusory Realm to fight the bosses once again with limited equipment. Defeating each boss completes the respective Champion's Song quest and rewards link with a shortened cooldown on that Champion's power.
When Link completes all four Champions' songs, he is requested to head back to the Shrine of Resurrection. As soon as the Sheikah Slate is placed back in its pedestal, the entire chamber starts to descend, bringing Link to the Final Trial. It works just like the four Divine Beast dungeons, and upon completing it and defeating the final boss Monk Maz Koshia, Link is rewarded with the Master Cycle Zero. Link encounters Kass again as he exits the Shrine of Resurrection. Kass plays the completed Champions' Ballad and gives Link the Picture of the Champions.